Servo displacement and speed control system



"if, JJ e 8, 1965 w. w. Bam. .n.1

SERVO DISPLACEMENT BND SPEED gG'QIIIIEICFL SYSEEM Filed mmh 126;, 1962United States Patent O The present invention generally relates to servocontro-l apparatus and, more particularly, to a system adapted forautomatically controlling the angular displacement and rotational speedof a servornechanisrn output shaft.

Displacement land speed control servomechanisrns have been proposed fora wide variety of applications. Varying degrees of precision ofdisplacement and speed control are required depending upon theparticular usage. For example, automatic machine tool controlapplications for the fabrication of precision cams demand close controlof tool displacement and tool feed rate so as to meet strict contour andsurface iinishing speciiications.

Precise control applications generally call for digital rather than.analog instrumentations, Accordingly, digitalzed servomechanisms havebeen proposed for the automatic control of precision machine tools. Thegeneral practice is to control tool displacements independently of toolfeed rates by means of separate command signals. Such practice tends tofurther complicate the already relatively complex digital machinecontrol system. Complexity, in turn, is integrally associated withequipment cost and reliability considerations. vlit is always desirable,or course, to achieve the desired precision without undue complexity andthus Awithout compromising cost and reliability.

it is the principal yobject of the present invention to provide simplieddigital apparatus for controlling the displacement output of aservo-mechanism.

Another object is to provide digital apparatus for controlling thevelocity output `of a servomechanism.

A further object is to provide automatic digital apparatus `forcontrolling the displacement and speed parameters of a servomechanism bymeans of the same control signal.

An additional vobject is to provide a digital servo displacement andspeed control system for a machine tool.

Another object is to provide Ia simplified digital automatic controlsystem for simultaneously controlling the tool displacements and thetool `feed rates of a machine tool along two perpendicular axes of .tooltravel.

A further object is to provide means responsive to digital controlsignals derived from a single tape for simultaneously controlling thedisplacements and tool feed rates of a machine tool along twoperpendicular axes.

These and other objects of the present invention, as will appear from areading of the following specification, are achieved in a preferredembodiment by the provision of an automatic machine tool control systemresponsive to divital command data stored on a single tape. Each blockof control data on the `tape comprises two binary numbers. One `of thenumbers represents a desired displaceyment increment through which thetool is to be moved in a iirst direction. The second number represents asecond displacement increment through which the tool is to be moved in asecond direction perpendicular to said rst direction.

A single tape reader is employed to read each of the numbers insuccession as the tape is advanced at a predetermined rate. Binarysignal representations `of the numbers are then stored in respectiveregisters, each register storing displacement increment data pertainingto a respective direction or axis along which the machine tool is to becontrolled.

M8572 Patented .lune 8, i965 ICC Each binary signal is applied to arespective pulse time position modulator which produces la fixed numberof pulses whose .time spacing is determ-ined by the value of the digitalnumber read from the tape. The time modulated output pulses, in turn,actuate a sampling gate which also -receives a feedback sine wave signalhaving a phase representing the actual machine tool displacement.

The nominal spacing between the successive sampling pulses .is preciselyequal to the time separation between the successive crossover points ofthe feedback sine Wave in the event that no machine tool displacementincrement is desired. in order to produce tool displacement from presentposition, `the spacing between the successive sampling pulses is variedfrom the nominal amount whereby the sampling pulses no longer coincidewith the sine wave crossover points. An output (error signal is producedby the sampling gate having .an amplitude proportional to the degree ofnon-coincidence and having a polarity dependent on the sense of saidnon-coincidence. The error vsignal is applied to a servo motor whichsimultaneously moves the machine tool to its new position and varies thephase of the feedback sine Wave Vuntil the error signal from thesampling gate is reduced to zero. If desired, the error signal may beadditively combined with another signal which is the analog of thedisplacement increment command. The combined signal, when applied to the`servo motor, causes the machine tool to be driven more precisely inaccordance with la desired velocity to a desired position.

An important feature of the invention is the manner in which thegeneration of servo command data is synchronized to the .operation ofthe data tape reader. As previously mentioned, a fixed number of Vtimemodulated pulses is produced in response to each number read from thetape. The tape, however, is advanced at a constant rate so that the samelength of time is available to generate the xed number of time modulatedpulses between each successive reading of the tape. The interval betweensuccessive advances of the tape is made substantially equal to the timerequired to generate the fixed number of command pulses having thenominal spacings established for the case where no displacementincrement is required :of the servo output shaft.

No synchronization problem arises when the interval between thesuccessive command pulses is reduced in response to the tape data. Onthe other hand, insuicient time is alloted for generating the successivecommand pulses when the tape data calls for a servo displacementincrement of an opposite sense. -In the latter case, the inventionprovides for the automatic reduction of the time interval l1oetween thelast occurring'pulse-s of each set of command pulses so that the samelixed number of command pulses is produced for every successive advanceof the tape. y

For a more complete understanding of the present invention, referenceshould be had to the following specification and to the sole iigurewhich is a simplified block diagram of a preferred embodiment.

yReferring to the figure, the numeral 1 generally represents `a magneticdrum source of system synchronizing pulses. VEach of tracks 2, 3, 4 and5 of drum 1 produce a total `of 2,048 pulses `for each revolution of thedrum. When the drum is rotated at 80 revolutions per second, forexample, each track produces 163,840 pulse-s per second. Moreover, eachof tracks 2, 3, d, and 5 is arranged to produce pulses phase displacedby electrical degrees from the pulses or" the adjacent track. The pulsesread out of tracks Z, 3, land S are combined in summing circuit 6 toproduce a series .of pulses on output line 7 having a repetition rate of655,360 pulses per second.

Drum l. is driven by a synchronous motor 8 which,

in the illustrative case, rotates at a speed of 80 revolutions persecond. Motor t3 also drives a two-phase generator 9 which produces onlines 1t) and 11, respectively, a pair of output sine waves at thesamefrequency of 80 cycles per second but phase displaced relative to eachother by 90 electrical degrees. Each of the output signals is applied asreference excitation to a resolver of a respective tool control axis.Only the apparatus required for controlling the machine tool along oneof the axes is represented in the drawing. Substantially identicalapparatus is required for the other control axis.

The 80 cycle per second sine Wave appearing on line lil is applied toresolver 12. The rotor of resolver 12 is driven by shaft 13synchronously with the rotation of the tool drive 14 which controls themachine tool generally represented by load 15) along one axis ofmovement. Resolver 12 produces on line 16 a sine Wave nominally at afrequency of 80 cycles per second and having a phase determined by theposition of shafts 13 and 14. The phase shifted sine Wave on line 16 isapplied to a sampling gate 17. Gate 17 is rendered conductive bysampling pulses applied via line 18 in a manner to be described later.

In the event that the sampling pulses of line 13 concur with the zerocrossover points of the phase shifted sine Wave of line 16, no outputsignal is produced by gate 17. On the other hand, where the samplingpulses occur at other than the crossover times, a series of pulses isproduced on output line 19 having an amplitude proportional to thedegree of non-coincidence and a polarity determined by the sense of thedisplacement of the sampling pulses from the zero crossover points ofthe phase shifted sine wave of line 16. The output pulses, if any, online 19 are applied via pulse integrating filter 20 to modulator 61.Filter 2@ may be a conventional boxcar detector. Negleeting, for themoment, the purpose of summing circuit ed, the modulated signal outputfrom modulator 61 is coupled by circuit 66 and amplifier 21 to motor 22.In operation, motor 22 is driven in a direction and by an amount tocause the zero crossover points of the phase shifted sine wave of line16 to coincide with the sampling pulses on line .18 at the input tosampling gate 17.

The pulses appearing on line 7 at the output of summing circuit 6 areapplied to the input of a six stage binary counter 2.3. Counter 23 isadapted in a Vconven- Itional manner to receive signals via parallelinput lines 24 for ladvancing :the value of the binary number stored incounter 23. The magnitude of the numerical advances are determined inaccordance with digital data received by tape reader 25 from a commanddata source (not shown). Y

In the preferred embodiment, the data source is a perforated paper tapehaving successive blocks of binary command data. Each block of dataconsist of two rows of perforations. Each row provides for a seven placebinary number. Each number, in turn,represents a desired displacementincrement through which the machine tool is tto be moved along apredetermined axis of tool travel. One of the two binary numbers of eachblock of data determines tool travel along a first axis; the othernumber of each block of data determines tool travel along a second axisperpendicular to said first axis. As the tapeis advanced in response tothe pulses of line 26, the command data numbers for the respective axesare read in alternation. That is, every other number read by reader 25pertains to commands along the first axis whereas the interveningalternate numbers read by reader 25 pertain to commands along the secondaxis. The tape is advanced times per second in synchronisrn with thereference signals of lines 7 and 1Q by the pulses of line 26.

The series of pulse signals representing the first axis command dataread by reader 2S is applied. by parallel lines 27 to buffer register28. Binary signal representations of the second axis command data aremade available on parallel lines 29 for application to a secon-d machinetool channel (not shown) which is substantially identical to the commandchannel shown in the ligure. The seven bit binary signal representingthe data stored in register 23 is applied via parallel lines 3i) tofirst inputs of sampling gates 31. Gates 31 are actuated by triggerpulses appearing on line 32 to transfer the rst 6 bits of the binarysignal to respective stages of counter 23.

In operation, the value of the binary number stored in counter 23 isadvanced by the Value of one least significant bit in response to eachpulse applied by line 7. On the other hand, the value of the binarynumber in counter 23 is advanced abruptly by a controllable amount inaccordance with ythe value of the six bit binary signal appearing onparallel lines 24 each time that gates 31 are actuated. The value of thesix bit binary signal, in turn, is determined by the binary numberperforations on the tape which is read by reader 25. In the event thatthe value of the tape command number is zero, no signals are passed bygates 31 and counter 23 operates to divide the 655,360 pulses per secondrepetition rate of the pulses on line by a factor of 2G or 64 to producea series of output pulses on line 33, having the nominal repetition rateof 19,240 pulses per second.

I Assuming for the moment that the pulses of line 33 are directlyapplied to the input of conventional seven stage binary counter 3S andassuming further that the epetition rate of said pulses is 10,240 pulsesper second, counter 35 produces a series of output pulses on line 36 atthe repetition rate of 8O pulses per second. The output pulses of line36 are shaped in pulse forming circuit 37 and then applied as triggerpulses to sampling gate 17 via line 18. The phase delays in theapparatus is adjusted so that the sampling triggers of line 18 coincidewith the zero crossover points of the phase shifted sine wave of line 16in the assumed case where the value of the binary number derived fromthe perforated tape is zero.

As previously mentioned, the same predetermined number of timemodulatedsampling pulses (pulses of line 36) is produced in response to eachbinary number read from the perforated tape. In the case of thepreferred embodiment, it has been found convenient to produce eightsampling pulses in response to each tape number representation. This isaccomplished with the use of the logical decision circuits includingpulse counter 3S, ip-op 39, pulse delay circuit 40 and AND circuit 41.Counter 3S divides the repetition rate of the pulses on line 32 by afactor of 8. Flip-flop 39 is placed into a condition for actuating ANDcircuit d1 by each pulse appearing on line 43. Flip-flop 39 is placed inits other state to block AND circuit 41 by the output pulses of counter3S.

The pulses on line 43 are derived from drum 1 in a manner now to bedescribed. Track 44 of drum 1 produces one pulse for each drumrevolution. Thus, when the drum is rotated at revolutions per second,the pulses read out and appearing on line 45 recur at 80 pulses persecond repetition rate. The pulses of line 45 are applied to the inputof conventional three stage binary pulse counter 46 to produce with theaid of pulse former 47 a series of pulses having a repetition rate of 20pulses per second on line 26 and a series of pulses having a repetitionrate of l0 pulses per second on line 43.

`Before proceeding with a description of the remaining structure of thefigure, it is helpful to briefly consider the operation of the apparatusso far described. The tape is advanced at a 20 step per second rate.Inasmuch as the servo command data for the two perpendicular axes oftool travel are interleaved upon the tape, a given tool displacementcommand for one axis must be processed in 1/10 of a second before thenext command for that axis is read from the tape. The binary signalsrepresenting a given command are shifted out of buffer register 28 onceeach th of a second synchronously with one of the 10 cycle per secondpulses at the output of counter 46. These pulses, which mark the startof each data processing interval, are also applied via line 43 to ilipiiop 39 to render AND circuit 41 conductive. The second tool axis (notshown) is synchronized to the same pulses.

Counters 23 and 35 may be considered as comprising a singleV 13 stagepulse counter, the rst six stages of which are adapted to receive binarysignals from butler register 23 each time that gates 31 are actuated bya pulse on line 32 at the output of circuit 41. Assuming that no signalsare passed by gates 31, cascaded counters 23 and 3S divide the pulserepetition rate (655,360 pps.) of the pulses on line 7 by a factor of213 or 8,192 to produce a series of 80 cycles per second pulses on line36 at the output of counter 35. The S0 cycles per second pulses on line36 pass through conductive AND circuit 41 and line 32 to activate gates31 eighty times per second. inasmuch as the binary signal stored inregister 23 changes only ten times per second, it will be seen that eachnumber stored therein is transferred to counter 23 eight successivetimes before the number is changed.

It should be noted that the time spacing between the pulses produced online 36 depends upon the value of the number transferred from bufferregister 23 to counter 23. lf the number transferred is larger thanZero, the repetition rate of the pulses on line 7 will be divided by afactor smaller than 213 thereby decreasing the time separation betweenthe successive pulses of line 36. After eight successive pulses appearon line 36 during the data proc essing interval marked by a given pulseat the output of counter do, counter 38 resets hip ilop 39 to close ANDcircuit 41. A decrease in the spacing between the successive pulses online 36 causes sampling gatel to be actuated in advance of the zerocrossover points of the sine wave of line 16. The resulting error signaldrives motor 22 in a direction so as to advance the phase of the sinewave at that rate which causes the zero crossover points to concur withthe sampling pulses of line 13.

In order to drive resolver 12 and output shaft 14 in a directionopposite to that just described, it is necessary to increase the timeseparation between the successive pulses produced on line 36. Suchtime-modulation of the pulses of line 36 results from what might betermed a negative displacement increment command. As before, themagnitude or" the displacement is determined by the value of the six bitbinary number transferred from butler register via gates 31 to counter23. The sense of the displacement is determined by the value of theseventh bit which is transferred by gates 31 to line 43. When theseventh bit is a one, iiip iiop 49 is set to a condition which blocksAND gate 34. Flip iiop 49 is reset to open AND gate 34 by the iirstpulse appearing on line 33 and passing through normally conducting gate5l). Gate Sil is momentarily blocked each time that a pulse appears online 36.

If the sign of the displacement command is positive (the value of theseventh or sign bit being zero), AND gate 34 always conducts. lf thesign or the displacement command is negative (the value of the seventhdigit is unity), and gate 34 blocks each first pulse appearing on line33 but passes all of the subsequent pulses during each interval markedby successive pulses on line 36. The etlect is to lengthen the intervalbetween the successive pulses on line 35 by an amount proportional tothe twos complement of the six bit number transferred from butlerregister 28 to counter 23 by gates 3l. Thus, a negative displacementcommand lengthens the time separation between the pulses on line 36whereby sampling gate 17 is actuated at times subsequent to theoccurrences of the zero Crossovers of the phase shifted sine waveproduced on line 16. An error signal of appropriate polarity isgenerated by gate 17 to drive motor 22 in the proper direction so as torestore coincidence between the sampling pulses and the zero crossoverpoints.

It will be observed that the lengthening of the repetition interval ofthe pulses on line 3d from the nominal interval of l/SO of a second (inthe case of negative displacement commands) causes the total interval ofeight successive pulses on line 36 to exceed 1/10 of a second. Thus, theoccurrence of eight successive pulses on line 36 is not always areliable basis for actuating gates 31 the required eight times beforethe number stored in buffer register 28 is changed to a new commandvalue. Accordingly, AND circuit 51, pulse former SZ and OR circuit 53are provided to makepcertain that gates 31 always are properly actuatedduring a given data processing interval. AND circuit 51 is conditionedfor conduction each time that it receives a gating pedestal from theoutput of AND circuit Sd via line 5S. Circuit 54, in turn, is connectedto each of the three stages of counter dfi and produces an intervalmeasuring or gatinrl pulse pedestal on line 55 having a leading edgeoccurring 1/0 of a second prior to the production of each l0` cycles persecond pulse on line 43. That is, AND circuit S4 becomes conductive eachtime that the count in counter 46 is one less than the value required toproduce an output pulse on line d3.

i The gating pulse pedestal on line 55 actuates circuit 51 and permitsthe next occurring output pulse of stage C11 of counter 35 to passthrough OR gate 53 and AND circuit il in lieu of the eighth pulse ofline 36; This ac- Vtion occurs whenever tzt-negative displacement isbeing executed at which times said eighth pulse occurs too late toactivate gates 31 before a new number is inserted in buffer registerV23. In this mannengates 31 are always activated exactly eight successivetimes either by eight successive pulses on line 36 (for positivedisplacements) or by a combination of the pulses of line 35 and theoutput pulses of stage C11 of counter 35 (for negative displacements).lrrespective of whether a positive or a negative displacement command isbeing executed by the machine tool control system, however, it is alwaysthe pulses appearing on line 36 which activate sampling gate 17.

In summary, each displacement increment represented by theV punched tapedata is converted into a xed number of time-modulated pulses on line 36.The time spacing between each of the time-modulated pulses in each groupis determined by the value of a respective displacement in-V crementcommand. The pulses on line 36 Ysample the phase shifted sine-wave online 16 to generate an error signal at the output of gate 17 of propermagnitiude and sense to control the displacement and the time rate ofchange orn displacement of the output shaft of servo motor 22. Servomotor 272 drives the machine tool along a predetermined one of its twocoordinate axes of tool travel. The machine tool is simultaneouslycontrolled along the other of its coordinate axes by a separate dataprocessing channel substantially the same as the one represented in thedrawing.

The displacement commands are determined independently for each channel.For this reason, the single tape is stepped at a predetermined ratesynchronously with the rotation of drum 1 rather than at a ratedependent upon the value of the command data for either channel. Thus,it is necessary that each channel complete the processing of itsrespective data in a fixed interval of time, determined by the rotationof drumV 1. Alternatively, separate tapes and tape handling equipmentcould have been provided for the independent control of each datachannel. An important feature of the invention lies in the eliminationof a second tape and second tape handling equipment by using a singletape and single tape handling equipment for deriving the command datafor both of the data processing channels.

The valueof the number represented by the binary signals on lines 3) atthe output of butler register 28 determines the incremental displacementof the shaft14 of driving motor 22. Inasmuch as the signals on lines Silare applied to counter 23 at times and at rates which are determined bythe successive actuations of gates 31,`

it can be seen that the displacement as well as the velocity of shaft 14are controlled. There vvill be, however, a finite displacement trackingerror due to the fact that a finite error signal is required at theoutput of sampling gate 17 to drive motor 22 unless special provision ismade. Such tracking error may be eliminated by introducing a velocitycontrol term in the servo error signal path. This can be accomplished inaccordance With the present invention by the simple addition of summingcircuit 6@ and digital to analog converter 62.

Each of the digital signals at the outputs of gates 3l is applied toconventional converter 62 which also receives the same illustrative 400cycle excitation as is applied to motor 22 and modulator 6i. Converter62 producesin a conventional manner a 400 cycle signal on line 63 havingan amplitude representing the analog value of the binary signal on lines24 and a phase determined by the sign digit present on line 48. The 400cycle analog voltage is applied to summing circuit 60 wherein it isadditively combined with the 400 cycle error signal output of modulator6l. and then applied via amplifier 21 to motor 22. The signalintroducedby line 63 and circuit di) drives motor 22 at the rate commanded by thedigital signals appearing on lines 30. The servo loop acts to bring theCrossovers of the feedback sine wave of line 16 into coincidence withthe sampling pulses of line 18 whereby both the displacement and thevelocity of shaft 14 are controlled precisely.

W'hile the invention has been described in its preferred embodiments, itis understood that the Words which have been used are Words ofdescription rather than of limitation and that changes Within thepurview of the appended claims may be made Without departing from thetrue scope and spirit of the invention in its broader aspects.

What is claimed is:

1.v Generating meanstfor producing a fixed number of output pulses timespaced from each other in accordance with the value of a numberrepresented by a digital signal, said generating means comprising:

a source for producing timing pulses and interval measuring pulses, thefrequency of said timing pulses being higher than the frequency of saidmeasuring pulses,

a source for producing said digital signal,

a pulse counter comprising a first group and a second group of countingstages connected in cascade,

the first stage of' said first group being connected to receive saidtiming pulses and each stage of said first group being connected toreceive a respective bit of said digital signal, each of said timingpulses changing the value of the count 4in said first group by the samepredetermined amount, said digital signal changing the value of thecount in said first group by an amount determined by the value of thenumber represented by said digital signal,

first actuable means responsive to a control signal for applying saiddigital signal to said first group,

second actuable means connected to pass a signal from one of the stagesof said second group when said second actuable means is actuated, saidinterval measuring pulses being applied to and actuating said secondactuable means,

means for summing the output of said second actuable means and theoutput of the last stage of said second group to produce said controlsignal, said last stage of said second group producing said outputpulses,

third actuable means for selectively connecting the output of saidsumming means to said first actuable means when said third actuablemeans is actuated,

and means for counting the outputs from said third actuable means toproduce an output signal when a fixed number of said outputs haveoccurred, said output signal deactuating said third actuable means.

2. Generating means for producing a fixed number of output pulsestimespaced from each other in accordance CII with the value of a numberrepresented by a digital signal, said generating means comprising:

a source for producing timing pulses and interval measuring pulses, thefrequency of said timing pulses being higher and harmonically related tothe frequency of said measuring pulses,

a source for producing said digital signal,

a pulse counter comprising a first group and a second group of countingstages connected in cascade,

the first stage of said first group being connected to receive saidtiming pulses and each stage of said first group being connected toreceive a respective bit of said digital signal, the last of said secondgroup producing said Output pulses, each of said timing pulses advancingthe value of the count in said rst group by the same predeterminedamount, said digital signal advancing the value of the count in saidfirst group by an amount determined by the value of the numberrepresented by said digital signal,

actuable means for applying said digital signal to said first group,

a first gating circuit connected to pass a signal from one of the stagesof said second group when said first gating circuit is actuated, saidinterval measuring pulses being applied to and actuating said firstgating circuit,

means for summing the output of said first gating circuit and the outputof said last stage of said second group,

a second gating circuit for selectively connecting the output of saidsumming means to said actuable means when said second gating circuit isactuated,

and means for counting the outputs from said second gating circuit forproducing a control signal when a fixed number of outputs have occurred,said control signal deactuating said second gating circuit.

3. Generating means for producing a fixed number of output pulses timespaced from each other in accordance with the algebraic value of anumber represented by a digital signal, said generating meanscomprising:

a source for producing timing pulses and interval measuring pulses, thefrequency of said timing pulses being higher than the frequency of saidmeasuring pulses,

a source for producing said digital signal,

a pulse counter comprising a first group and a second group of countingstages,

the last stage of said iirst group being selectively conuected to thefirst stage of said second group in laccordance with the sign of thenumber represented by said digital signal,

the first stage of said first group being connected to receive saidtiming pulses and each stage of said rst group being connected toreceive said digital signal, each of said timing pulses changing thevalue of the count in said first group by the same predetermined amount,said digital signal changing the `value of the count in said first groupby an amount determined 'oy the magnitude of the number represented bysaid digital signal,

first actuable means responsive to a control signal for applying saiddigital signal to said first group,

second actuaole means connected to pass a signal from one of the stagesof said second group when said second actuable means is actuated, saidinterval measuring pulses being applied to and actuating said secondactuable means,

means for summing the output of said second actuable means and theoutput of the last stage of said second group to produce said controlsignal, said last stage of said second group producing said outputpulses,

third actuable means for selectively connecting the output of saidsumming means to said first actuable means when said third actuablemeans is actuated,

and means for counting the outputs from said third actuable means toproduce an output signal when a fixed number of said outputs haveoccurred, said output signal deactuating said third .actuable means.

4. In a servo system wherein the serv-o error signal is produced bycomparing the phase of a servo feedback signal with the occurrences of aseries of command pulses, said command pulses being time spaced fromeach other in accordance vwith the value Iof a number represented by adigital signal, means for generating said command pulses comprising:

a source dior producing timing pulses and interval meas- Iuring pulses,the frequency of said timing pulses being higher than the frequency ofsaid measuring pulses,

a source for producing said digital signal,

a pulse counter comprising a iirst group and a second group of countingstages connected in cascade,

the iirst stage of said iirst group being connected to receive `saidtiming pulses and each stage of said first group being connected toreceive said digital signal, each of said timing pulses changing thevalue of the count in said first group by the same predetermined amount,said digital signal chang-ing the value of the count in said iirst.group by `an amount determined bythe value of the number represented bysaid digital signal,

a first gating circuit respon-sive to a control signal for applying saiddigital signal to said iirst group,

ya second gating circuit connected to pass a signal from one of thestages of said second group when said second gating circuit is actuated,said interval measuring pulses being applied to and actuating saidsecond gating circuit,

means for summing the output of said second gating circuit and theoutput of the last stage of said second ygroup to produce said controlsignal, said last stage of said second group producing said commandpulses,

a third gating circuit for selectively applying the output of saidsumming means to said first gating circuit when said third gating.circuit is actuated,

and means for counting the outputs from said third gating circuit toproduce an output signal when a =xed number of said outputs haveoccurred, said output signal deactuating said third gating circuit.

5. In a servo system wherein the servo error signal is produced bycomparing the phase of a servo feedback signal with the occurrences of aseries of command pulses, said command pulses being time spaced fromeach other in accordance with the algebraic value of a numberrepresented by a digital signal, means -for generating said pulsescomprising:

a source for producing timing pulses and interval meassuring pulses, thefrequency of said timing pulses being higher than the frequency of saidmeasuring pulses,

a source for producing said digital signal,

a pulse counter comprising a first group and a second group of countingstages,

the last stage of said first group being selectively connected to lthefirst stage of said second group in ac- -cordance with the sign of thenumber represented by said digital signal,

Ithe first stage of said first group being connected to receive saidtiming pulses and each stage of said iirst group being connected toreceive said digital signal, each of said t-iming pulses changing thevalue of the count in said first -group by the same predeterminedamount, said digital signal changing the value of the count in said rstgroup by an amount determined by the magnitude of the number representedby said digital signal,

a first gating circuit responsive to a Icontrol signal for applying saiddigital signal to said first group,

a second gating circuit connected to pass a signal from one of thestages of said second group when said second gating circuit is actuated,said interval measuring pulses being applied to and actuating saidsecond gating circuit,

means Vfor summing the output of said second gating circuit land theloutput of the last stage of said second :group to produce said controlsignal, said last stage of said second group producing said outputpulses,

a third gating circuit for selectively applying the output of .saidsumming means to said first gating circuit lwhen said third gatingcircuit is actuated,

and means for counting the outputs from said third gating circuit toproduce an output signal when a fixed number of said outputs haveoccurred, said output signal deactuating said third gating circuit.

6. A servo system wherein an error signal is produced by comparing thephase of a feedback signal with the occurrences of a series of commandpulses and wherein said error signal is combined with a se-cond signalto control the phase and the frequency of said feedback signal, saidsystem comprising:

a source for producing timing pulses synchronously related to saidfeedback signal,

a source for producing a digita-l signal,

a pulse counter comprising la first group and a second :group ofcounting stages connected in cascade,

the first stage of `said first group being connected to receive saidtiming pulses yand each stage of said iirst 'group being connected t-oreceive said digital signal, the last stage of said second groupproducing said command pulses, each of said timing pulses changing thevalue of the count in said first group by the same predetermined amount,said digital signal changing .the value of the count in said lirst groupby an amount determined by the value of the number re lpresented by saiddigital signal,

first actuable means responsive to a control signal for yapply-ing saiddigital signa-l to said first group,

second actuable ,means :for applying the output of the last stage ofsaid second group to said first actuable means when said secondactuable' means is actuated,

means for counting the outputs from said second actuable Ameans .toproduce an youtput signal When a fixed number .of said outputs haveoccurred, said output signal deactuating said second actuable means,

means coupled to receive said digital signal for producing .said secondsignal having a characteristic related to said value of said numberrepresented by `said digital signal, and

means for combining said second signal and said error signal.

References Cited bythe Examiner UNITED STATES PATENTS 2,945,183 7/ 60Hartke et al 328-48 2,994,790 8/60 Delaney 328-42 X 3,002,151 9/ 61Broderick 328-42 X 3,015,806 1/62 Wang et al. 328-34 X 3,044,065 7/6-2Barney 328-42 X ARTHUR GAUSS, Primary Examiner.

MALCOLM A. MORRISON, Examiner,

1. GENERATING MEANS FOR PRODUCING AS FIXED NUMBER OF OUTPUT PULSES TIMESPACED FROM EACH OTHER IN ACCORDANCE WITH THE VALUE OF A NUMBERREPRESENTED BY A DIGITAL SIGNAL, SAID GENERATING MEANS COMPRISING: ASOURCE FOR PRODUCING TIMING PULSES AND INTERVAL MEASURING PULSES, THEFREQUENCY OF SAID TIMING PULSES BEING HIGHER THAN THE FREQUENCY OF SAIDMEASURING PULSES, A SOURCE FOR PRODUCING SAID DIGITAL SIGNAL, A PULSECOUNTER COMPRISING A FIRST GROUP AND A SECOND GROUP OF COUNTING STAGESCONNECTED IN CASCADE, THE FIRST STAGE OF SAID FIRST GROUP BEINGCONNECTED TO RECEIVE SAID TIMING PULSES AND EACH STAGE OF SAID FIRSTGROUP BEING CONNECTED TO RECEIVE A RESPECTIVE BIT OF SAID DIGITALSIGNAL, EACH OF SAID TIMING PULSES CHANGING THE VALUE OF THE COUNT INSAID FIRST GROUP BY THE SAME PREDETERMINED AMOUNT, SAID DIGITAL SIGNALCHANGING THE VALUE OF THE COUNT IN SAID FIRST GROUP BY AN AMOUNTDETERMINED BY THE VALUE OF THE NUMBER REPRESENTED BY SAID DIGITALSIGNAL, FIRST ACTUABLE MEANS RESPONSIVE TO A CONTROL SIGNAL FOR APPLYINGSAID DIGITAL SIGNAL TO SAID FIRST GROUP, SECOND ACTUABLE MEANS CONNECTEDTO PASS A SIGNAL FROM ONE OF THE STAGES OF SAID SECOND GROUP WHEN SAIDSECOND ACTUABLE MEANS IS ACTUATED, SAID INTERVAL MEASURING PULSES BEINGAPPLIED TO AND ACTUATING SAID SECOND ACTUABLE MEANS, MEANS FOR SUMMINGTHE OUTPUT OF SAID SECOND ACTUABLE MEANS AND THE OUTPUT OF THE LASTSTAGE OF SAID SECOND GROUP TO PRODUCE SAID CONTROL SIGNAL, SAID LASTSTAGE OF SAID SECOND GROUP PRODUCING SAID OUTPUT PULSES, THIRD ACTUABLEMEANS FOR SELECTIVELY CONNECTING THE OUTPUT OF SAID SUMMING MEANS TOSAID FIRST ACTUABLE MEANS WHEN SAID THIRD ACTUABLE MEANS IS ACTUATED,AND MEANS FOR COUNTING THE OUTPUTS FROM SAID THIRD ACTUABLE MEANS TOPRODUCE AN OUTPUT SIGNAL WHEN A FIXED NUMBER OF SAID OUTPUTS HAVINGOCCURED, SAID OUTPUT SIGNAL DEACTUATING SAID THIRD ACTUABLE MEANS.